Earyotic genomes contain not only stable genes, but also genetic elements that can be mobilized through a process of transposition. These transposable elements are often maintained in high copy number and their movement has been postulated to play a role in chromosome instability and mutagenesis. In particular L1, a member of the family of transposable elements known as retroposons, has an estimated copy number of 100,000/mammalian genome, and 10% of these may be capable of movement. Thus, substantial instability could result from L1 activation. To address this issue we are developing two experimental model systems that use phenotypic identification to quantitate transpositional events with the aim of determining what genetic factors, chemicals, or other environmental insults lead to retropson transposition. The transposable elements used will be marked genetically, such that transposition of the element can be identified immediately after the event. In one system, using the retroposon HeT-A in Drosophila, the white gene containing an inverted intron is used as a genetic marker. Transposition will be monitored in vivo. This system is partially developed and will be used to design a similar system in the mouse. In the mouse, GFP will be used as the genetic marker, and transposition of an L1 element will be monitored in vitro.